1. Field of the Invention
The present invention concerns a closed combustion system employing recirculated exhaust gas, for internal combustion engines, external fuel combustion furnaces, gas turbines, etc. In particular, this invention relates to a closed combustion system as outlined above which is arranged so that, while preventing the inflow of ambient air into the combustion system, there is recirculated to the combustion chamber the exhaust gases which contain no nitrogen taken from the atmosphere. The recirculated exhaust gases consist mainly of water vapor and carbon dioxide, both of which are inert gases. Pure oxygen and the fuel which are required for effecting the combustion are fed continuously into the system while also being sealed off from the atmosphere.
2. Description of the Prior Art
A certain level of development has been achieved in making exhaust gases clean with respect to those constituents, namely, hydrocarbons HC and carbon monoxide CO, which have not completely oxidized in the combustion process in internal combustion engines and fuel combustion furnaces. However, no satisfactory solution has been found to the problem of eliminating from the exhaust gases nitrogen oxides NO.sub.x, or so-called oxidants, NO.sub.x and SO.sub.x, mainly coming from nitrogen and sulfur constituents of the fuel burned in the closed system. Various efforts are being paid in various fields to find means to accomplish this goal.
In particular, the oxidants NO.sub.x, or NO.sub.x plus SO.sub.x, are measured usually in terms of p.p.m. and they are contained in the exhaust gases in very small amounts as compared with CO and HC which are present in the exhaust gases in such large amounts that they are usually measured in terms of volume percent. However, these oxidants constitute major ingredients of poisonous photochemical smogs and they play a great part in causing physiological injury to living things such as eye irritation, headache, cough, vomiting, and so on in human beings. Therefore, the amounts of such oxidants permitted in exhaust gases are severely limited by Environmental Protection Agency laws and regulations. Thus, vigorous research efforts are being made in various countries in order to prevent the generation, or to effect removal, of such oxidants.
As one method for producing cleaner exhaust gases, there is known the exhaust gas recirculation method. Exhaust gases are generally composed of water vapor (H.sub.2 O), carbon dioxide (CO.sub.2), carbon monoxide (CO), unburned fuel (HC), oxidants (NO.sub.x and SO.sub.x) and nitrogen (N.sub.2). These constituents of exhaust gases are generated when hydrocarbon fuels are burned with air.
In the known exhaust gas recirculation method, exhaust gases having the aforesaid ingredients are recirculated to the combustion apparatus. More specifically, this known method comprises feeding back the exhaust gases to the combustion apparatus to dilute or reduce the concentration of N.sub.2 which enters therein as an ingredient of incoming air and to lower the temperature at the time of combustion, thereby to reduce the production of NO.sub.x. However, in such a process, the amount of O.sub.2 supplied, which is indispensable for the combustion of the fuel and which must be taken from the incoming air, may be insufficient if the amount of recirculated exhaust gas is too large. Accordingly, the amount of exhaust gases which are recirculated is not permitted to be more than about 10-15 vol. %, based on the amount of incoming air. Recirculation of exhaust gases in excess of this range will result in a shortage of O.sub.2 in the combustion chamber and no combustion or poor combustion will take place. Thus, according to the known exhaust gas recirculation method, the volume of recirculated exhaust gases is quite limited, i.e. on the order of only 10-15 vol.% of the exhaust gases generated, and the remainder of the exhaust gases have to be discharged as waste gases into the atmosphere. Thus, discharge of the greater part of the oxidants NO.sub.x or NO.sub.x plus SO.sub.x into the atmosphere is still taking place at present.
If there is provided a so-called closed type system which discharges substantially no exhaust gases to the outside atmosphere, but rather recirculates all of these exhaust gases completely within the combustion system, then there would be practically no exhaust gases and accordingly no air pollution will arise due to exhaust gases discharged from ordinary internal combustion engines and external fuel combustion furnaces.
However, in order that combustion may take place smoothly and continuously in this closed combustion system, it is necessary to feed not only the fuel but also oxygen to the combustion apparatus from outside the system.
By performing combustion in such a manner that inflow of air into the combustion system is possible for only a very short time at the starting of the combustion operation, and inflow of air is prevented after the combustion operation has become stable and in its place there is recirculated into the combustion system a gaseous exhaust phase which does not contain nitrogen taken from the atmosphere at all, i.e. it consists mainly of water vapor and carbon dioxide, both of which are inert gases generated by fuel combustion, and further that fuel and pure oxygen which are necessary for the combustion are continuously fed in while excluding inflow of air, it will be understood that combustion will be carried out continuously in a closed system. In this mode of combustion, an appreciable amount of nitrogen is not present in the actuating gases so that no oxidants will be generated at all. However, as the combustion continues, the fuel (HC) will be oxidized and will be transformed into water vapor and carbon dioxide. Therefore, when combustion in a closed type combustion apparatus is continued, the amount of exhaust gases will increase as time goes by, causing the pressure in the closed type combustion apparatus to increase progressively. It is necessary, therefore, to provide means for maintaining the pressure of the gas stream fed into the combustion chamber at about 1 atmosphere pressure (absolute) by an appropriate device such as a safety (pressure release) valve.